1. Field of Invention
The invention relates to an automatic transmission and, more particularly, to a technology for supporting a thrust force generated at a change gear element by a power transmission in the gear train.
2. Description of Related Art
In addition to ensuring drivability of a vehicle, there is a demand for multi-staging of an automatic transmission for a vehicle in order to improve the consumption in fuel indispensable to energy saving. To respond to such a request, a change gear mechanism is changed from a conventional forward four-stage transmission to a five-stage transmission. Smaller elements of a gear train per gear change stage and simplification of the mechanism are required to realize further multi-staging in a limited vehicle loading space, requiring tremendous efforts to ensure the proper gear train structure. To accomplish this, there has been proposed a gear train for achieving forward six-stage and reverse one-stage transmission using a planetary gear set consisting of minimum change gear elements in U.S. Pat. No. 5,106,352 which shows a generic automatic transmission. The gear train according to the patent is characterized in that multi-stage gear change is achieved in combination with a reduction planetary gear and a planetary gear set for outputting a change gear rotation by defining the reduction rotation from the reduction planetary gear as an input.
A gear train structure according to the above proposal is principally reasonable in the number of change gear elements per gear change stage required and the required number of clutches and brakes; and, however, there is a practical problem to be addressed and corrected. In particular, in an automatic transmission for a vehicle, generally a helical gear is used for a sun gear, a pinion, and a ring gear that constitute a planetary gear in order to reduce gear noises. An advantage of such a structure is that the gear rates of the respective gears are increased by the helical gear, making it possible to reduce gear noises. In contrast, there is a problem that a thrust force contrary to the ring gear and sun gear, respectively, is generated.
The above patent does not address the problem in the thrust force or its solution. When the concepts, known to those of general skill, as employed in a conventional automatic transmission, are extended and applied to the above proposed gear train, the result is a method for disposing a center support between a reduction planetary gear (a single planetary gear set) and a planetary gear set (complex planetary gear set), thereby subjecting the center support and a case to the thrust force generated at the respective planetary gear sets by the power transmission so as not to interfere with each other. If such a structure is adopted, the axial dimensions of a transmission are increased by the arrangement of the center support. Thus, in particular, there is a problem that the ability to mount the gear on a vehicle is degraded because a large-scaled mechanism due to the multi-staging is unavoidable in a multi-stage transmission.
The present has been achieved to solve the above mentioned problem. It is one object of the invention to provide an automatic transmission capable of supporting a thrust force acting on a reduction planetary gear and a planetary gear set, respectively, without increasing axial dimensions of the transmission.
To achieve the above mentioned object, according to the invention, there is provided an automatic transmission for achieving a multi-stage gear change by a reduction planetary gear coupled to an input shaft and a fixed reaction force element so as to output a reduction rotation to an output element and a planetary gear set that outputs a rotation with a variable speed depending on the shift position upon input of the reduction rotation from the reduction planetary gear, in which a torsion direction of the helical teeth of the respective elements is set such that that a direction of the thrust force of the one element of the reduction planetary gear and a direction of the thrust force of the one element of the planetary gear set are different from each other during driving at one of a plurality of gear speeds.
In the above described structure, it is effective that the torsion direction is set such that a thrust force generated at one element of the reduction planetary gear and a thrust force generated at one element of the planetary gear set face each other during driving at the first gear speed; a bearing subjected to thrust forces generated at the respective elements in common is provided in the common force transmission route; and the bearing is subjected to a differential thrust force between a thrust force acting on one element of the planetary gear set and a thrust force acting on one element of the reduction planetary gear.
In addition, in the above mentioned structure, the foregoing torsion direction is a direction in which the thrust force generated at the one element of the reduction planetary gear and the thrust force generated at the one element of the planetary gear set depart from each other during first gear driving; a bearing subjected to a thrust force generated at the respective elements is provided; and the bearing is subjected to the thrust force acting on the one element of the planetary gear set and the thrust force acting on the one element of the reduction planetary gear independently.
In any of the above described structures, it is effective that a reaction force element of the reduction planetary gear is fixed to a case, and a thrust force to be transmitted to the common force transmission route is transmitted to the case via the bearing.
In addition, in any of the above described structures, it is effective that a carrier of the planetary gear set is supported axially on the common force transmission route.
Further, in any of the above described structures, it is effective that the planetary gear set includes a first sun gear as one element thereof and a second sun gear independent of the first gear; and a bearing subjected to a thrust force acting on the second sun gear is disposed on a passage different from the common force transmission route.
Furthermore, in any of the above described structures, it is effective that the planetary gear set includes a first sun gear and a second sun gear independent of the first sun gear; and a bearing is disposed between the first and second sun gears.
Still further, in any of the above described structures, it is effective that the foregoing one element of the reduction planetary gear is a ring gear.
Furthermore, in any of the above described structures, it is effective that a reaction force element of the reduction planetary gear is fixed to an oil pump case; and
a thrust force generated at the foregoing one element of the reduction planetary gear is transmitted to the oil pump case via the bearing.
Moreover, it is effective that the reduction planetary gear consists of a sun gear formed as the reaction force element; a carrier coupled with the foregoing one element of the planetary gear set for supporting a pinion geared with the sun gear; and a ring gear geared with a pinion supported by the carrier as the foregoing one element of the reduction planetary gear coupled with the input shaft via a coupling member, in which the bearing is disposed between the sun gear and the coupling member.
It is effective that the foregoing one element of the planetary gear set is a sun gear, the coupling member is disposed between the reduction planetary gear and the planetary gear set; and a second bearing is disposed between the coupling member and the sun gear.
Moreover, it is effective that the planetary gear set is formed as a planetary gear set of Ravigneaux type consisting of long and short pinions supported by a carrier and geared with each other; a first sun gear geared with the long pinion; a second sun gear geared with the short pinion; and a ring gear geared with any one of the long and short pinions.
Further it is effective that the ring gear is geared with the short pinion.
Furthermore, it is effective that the ring gear is geared with the long pinion.
Still furthermore, it is effective that the long pinion is geared with the first sun gear at one end thereof, and the ring gear is geared with the other end of the long pinion.
Moreover, it is effective that the input shaft is further geared with a carrier of the planetary gear set via another coupling member; a third bearing is disposed between the sun gear of the planetary gear set and another coupling member; a fourth bearing is disposed between the case and another coupling member, and a thrust force of the ring gear of the reduction planetary gear is transmitted to a case via second, third, and fourth bearings during reverse driving.
In the structure according to a first aspect, during first gear driving in which a driving force is the greatest, and a load due to a thrust force is applied, the respective thrust forces of a reduction planetary gear and a planetary gear set are set in a direction different from (opposed to or opposite to) each other depending on a combination of the torsion directions of the helical teeth that are gear change elements. Thus, when these directions face with or are opposed to each other, a thrust force of one element of the planetary gear set acts to a common force transmission route in one direction, and a thrust force of one element of the reduction planetary gear acts thereto in the other direction. As a result, the invention is structured so as to subject the outside of the common force transmission route to a differential thrust force. In addition, when the above directions are opposite to each other, no thrust force acts in the common force transmission route. As a result, the invention is structured so as to subject the outside of the common force transmission route to the respective thrust forces. Therefore, the durability of members subjected to the thrust forces can be prevented from being lowered. In addition, the thrust forces generated respectively at the reduction planetary gear and the planetary gear set are directionally opposed to each other, and are buffered to each other, thus making it unnecessary to provide a center support for receiving the thrust forces, and making it possible to reduce axial dimensions of the transmission correspondingly.
Next, in the structure according to a second aspect, a thrust force acting on one element of a planetary gear set and a thrust force acting on one element of a reduction planetary gear are opposed or facing to each other, and are offset, thereby making it possible to reduce a thrust force applied to a bearing arranged outside of a common force transmission route. Thus, the bearing can be compactly structured and, further, axial dimensions of the transmission can be reduced.
In addition, in the structure according to a third aspect, during first gear driving in which a driving force is the greatest, and a load due to a thrust force is applied, a thrust force acting on one element of a planetary gear set and a thrust force acting on one element of a reduction planetary gear are applied independently without being buffered to each other, thereby making it possible to reduce a trust force applied to a bearing arranged outside of a common force transmission route. Thus, the bearing can be compactly structured and, further, axial dimensions of the transmission can be reduced.
In the case where a reaction force element of a reduction planetary gear is not fixed to a case, another thrust bearing must be provided between the case and the reaction force element of the reduction planetary gear, the reaction force element fixed by appropriate fixing means, and, thus, an increase in axial dimensions of the transmission is unavoidable. In contrast, in the structure according to a fourth aspect, a sun gear is integrally fixed to a case, thereby making it possible to have only the bearing subjected to a thrust force and, further, making it possible to reduce the axial dimensions.
At least one end of a carrier of a planetary gear set is required to be axially supported. In the case where the support is used outside of the force transmission route, a support member of the carrier is intervened on a thrust force transmission route and, thus, the number of thrust bearings is increased. In contrast, in the structure according to a fifth aspect, a carrier is supported on a common force transmission route, thus making it possible to reduce the number of thrust bearings.
In the structure according to a sixth aspect, a thrust force of a common force transmission route acts to a first sun gear of a planetary gear set. In contrast, a second sun gear can be structured so that a thrust force of the first sun gear or reduction planetary gear transmitting a common force transmission route does not act to the second sun gear. Therefore, a bearing receiving the thrust force generated at the second sun gear can be compactly structured for receiving only the thrust force of the second sun gear outside of the common force transmission route.
In the structure according to a seventh aspect, a thrust force generated at a first sun gear can be transmitted between the first and second sun gears via a bearing.
Thus, as is the case where the thrust force generated at the first sun gear is transmitted via a carrier of a pinion gear geared thereto, arrangement of a bearing due to the carrier intervened in the force transmission route for thrust force transmission can be eliminated, thereby making it possible to reduce the number of bearings in a common force transmission route.
In the structure according to an eighth aspect, a ring gear and a sun gear of a reduction planetary gear face each other against the thrust force applied thereto with a bearing interposed between the gears. A thrust force to be transmitted from the ring gear to the case via a bearing is offset in opposition to the sun gear. Thus, it is possible to reduce the load upon members, such as case for receiving the thrust force via the bearing, and prevent its durability from being lowered.
In the structure according to a ninth aspect, a thrust force applied to an oil pump case can be reduced, thus making it possible to prevent the durability of the oil pump case from being lowered.
In the structure according to a tenth aspect, a large-scale thrust bearing, abutted against a sun gear of a reduction planetary gear, is not needed because of the reduction in the above thrust force. In the case where a structure having a bearing abutted against a tooth bottom of the sun gear is adopted, there is no need for increasing the tooth diameter of the sun gear in order to ensure an abutment face, and an increase in radial dimensions of the reduction planetary gear can be prevented. In addition, even in the case where a structure having a bearing abutted with the tooth end of the sun gear is adopted, there is no need for thickening a bearing race in order to ensure rigidity. As a result, an increase in the axial dimensions of the force transmission route can be prevented.
In the structure according to an eleventh aspect, in the case where a thrust force of a sun gear of a planetary gear set is greater than a thrust force of a ring gear of a reduction planetary gear, the thrust force from the sun gear is transmitted to a first bearing via a second bearing; and a thrust force of a ring gear of a reduction planetary gear is transmitted to the second bearing in opposition to the thrust force of the sun gear. Therefore, the thrust force offset in a common force transmission route is received by the first bearing, and the bearing can be compactly structured.
Further in the structure according to a twelfth aspect, a planetary gear set of Ravigneaux type is employed, thereby making it possible to reduce axial dimensions of the planetary gear set by the common use of a carrier.
In the structure according to a thirteenth aspect, power is transmitted on a short pinion side that does not cause a rotational moment, thereby making it possible to prevent an occurrence of rotational moment on along pinion side and reduce a load applied to a carrier and a rotation support portion of the career.
In the structure according to a fourteenth aspect, radial dimensions can be reduced more significantly than in the case where a ring gear meshes with a short pinion.
In the meantime, in the case where a planetary gear set of Ravigneaux type is employed or in the case where a sun gear and a ring gear are geared with each other in opposition to a long pinion, if the thrust directions of the sun gear and ring gear are defined in opposition to each other, a thrust force is generated at the long pinion geared with the sun gear and ring gear in a departure direction because of its long pinion length. Further, a separate force is generated as a way force between the gears in gear engagement, and acts in a direction in which the gears face each other. As a result, moments due to a thrust force and a separate force act to a long pinion in a synergistic manner, and a load acting on a support portion of the long pinion is increased.
In contrast, in the structure according to a fifteenth aspect, a torsion angle is set so that a direction in the thrust force generated at the sun gear of the planetary gear set faces a direction in the thrust force generated at the ring gear of the reduction planetary gear. As a result, a thrust force different from that of the sun gear is generated at the ring gear of the planetary gear set, and this force acts in a direction in which a moment is offset. Therefore, a load applied to the support portion of the long pinion can be reduced.
In the structure according to a sixteenth aspect, a thrust force generated at a ring gear of a reduction planetary gear is received by a case via second, third, and fourth bearings in reverse driving. Hence, a clearance between members is decreased more significantly than that obtained in a method for dividing a thrust force via an input shaft, thus, making it possible to reduce axial dimensions.